Wide bandgap semiconductor switching device with wide area schottky junction, and manufacturing process thereof

ABSTRACT

A switching device including: a body of semiconductor material, which has a first conductivity type and is delimited by a front surface; a contact layer of a first conductive material, which extends in contact with the front surface; and a plurality of buried regions, which have a second conductivity type and are arranged within the semiconductor body, at a distance from the contact layer.

BACKGROUND

Technical Field

The present disclosure relates to a wide bandgap semiconductor switchingdevice, which has a wide area Schottky junction; further, the presentdisclosure relates to the process for manufacturing the aforementionedswitching device.

Description of the Related Art

As is known, there are today available so-called “Junction-barrierSchottky” (JBS) power diodes, also known as “Merged PiN Schottky” (MPS)diodes. According to this technology, a diode forms two distinctcontacts: an ohmic contact and a Schottky contact.

Generally, JBS diodes are of silicon carbide. In particular, asdescribed, for example, in the U.S. Patent Publication No. 2015/0372093in the name of the present applicant, a JBS diode includes asemiconductor body of silicon carbide of an N type, which is delimitedat the top by a front surface, over which a conductive layer extends,formed, for example, by titanium. Present within the conductive layer,and in contact with the front surface, is a plurality of conductiveregions, made, for example, of nickel silicide. Further, formed withinthe semiconductor body are wells of a P type, which extend from thefront surface of the semiconductor body so that each well contacts acorresponding conductive region. In this way, between each conductiveregion and the corresponding well an ohmic contact is created. Inaddition, between the conductive layer and the portions of semiconductorbody arranged between the wells, corresponding Schottky contacts areformed.

The JBS diodes described above substantially have, at the workingcurrent, the same voltage drop as a Schottky diode. Furthermore, inreverse biasing and in the proximity of breakdown, JBS diodes exhibit aleakage current comparable with the leakage current of a bipolar diode.In addition, the presence of the ohmic contact enables JBS diodes towithstand, in forward biasing, high currents, thanks to the fact thatthe bipolar junction is activated. However, the presence of the ohmiccontacts involves the need to align the conductive regions and thecorresponding wells precisely. Furthermore, the overall area of ohmiccontact is limited by the quality of the alignment. This limit reflectsupon the possibility of increasing the density of the wells. Inaddition, the presence of the wells causes a reduction of the usefularea for creation of the Schottky contact, with consequent reduction ofthe possibility of reducing the voltage drop across the diode, at theworking current.

BRIEF SUMMARY

Some embodiments of the present disclosure are a device and amanufacturing process that will overcome at least in part the drawbacksof the known art.

According to one embodiment the present disclosure, a wide bandgapsemiconductor switching device includes a body of semiconductormaterial, which has a first conductivity type and a front surface; acontact layer of a first conductive material, which extends in contactwith the front surface; and a plurality of buried regions. The buriedregions have a second conductivity type, are arranged within thesemiconductor body, and are spaced apart from the front surface, thecontact layer, and each other by portions of the body.

One embodiment the present disclosure is a manufacturing process thatincludes forming, within a body of semiconductor material having a firstconductivity type and a front surface, a plurality of buried regions ofa second type of conductivity, said buried regions being spaced apartfrom the front surface and each other by portions of the body; andforming, in contact with the front surface, a contact layer of a firstconductive material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present disclosure preferredembodiments thereof are now described, purely by way of non-limitingexample, with reference to the attached drawings, wherein:

FIGS. 1-4 are schematic cross-sectional views of embodiments of thepresent switching device;

FIGS. 5A-5E are schematic cross-sectional views of the embodimentillustrated in FIG. 1, during successive steps of a manufacturingprocess;

FIGS. 6A-6E are schematic cross-sectional views of the embodimentillustrated in FIG. 2, during successive steps of a manufacturingprocess;

FIGS. 7A-7E are schematic cross-sectional views of the embodimentillustrated in FIG. 3, during successive steps of a manufacturingprocess; and

FIGS. 8A-8E are schematic cross-sectional views of the embodimentillustrated in FIG. 4, during successive steps of a manufacturingprocess.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of the structure of a JBS diode,referred to in what follows as the diode 1.

In detail, the diode 1 comprises a semiconductor body 2, which is made,for example, of a wide bandgap semiconductor, such as for example amaterial chosen from silicon carbide (SiC), gallium arsenide (GaAs), andgallium nitride (GaN). In what follows, without this implying any lossof generality, it is assumed that the semiconductor body 2 is of siliconcarbide.

The semiconductor body 2 comprises a substrate 4 of an N+ type and afirst epitaxial layer 6 of an N type. The first epitaxial layer 6 isarranged on the substrate 4, with which it is in direct contact, and hasa doping level lower than the doping level of the substrate 4. Inaddition, the semiconductor body 2 is delimited at the top and at thebottom by a first surface S_(a) and a second surface S_(b),respectively, which are formed by the first epitaxial layer 6 and by thesubstrate 4, respectively.

The diode 1 further comprises a bottom contact layer 10, which is made,for example, of nickel silicide and extends underneath the secondsurface S_(b), in direct contact with the substrate 4.

The diode further comprises a conductive layer 12 and a topmetallization 14.

The conductive layer 12 extends over the first surface S_(a), in directcontact with the first epitaxial layer 6, and is of a metal, such as forexample a material chosen from nickel, titanium, and molybdenum, or anytransition metal.

The top metallization 14 extends over the conductive layer 12 and indirect contact with the latter. Furthermore, the top metallization 14 isof a metal material, such as for example aluminum, and has a thicknessgreater than the thickness of the conductive layer 12. For practicalpurposes, the top metallization 14 is designed to contact a conductiveelement, such as for example a so-called “lead”, in order to make itpossible to inject current into the diode 1 or draw off currenttherefrom. Consequently, the top metallization 14 is designed towithstand the mechanical stresses induced by contact with the conductiveelement.

The diode 1 further comprises a plurality of buried regions 20, of thesame semiconductor material as that of the semiconductor body 2, whichare located at a distance from the first surface S_(a) and thus do notcontact the conductive layer 12.

Without this implying any loss of generality, in the embodimentillustrated in FIG. 1, the buried regions 20 are of a P type and aresubstantially the same as one another. Further, the buried regions 20extend to approximately the same depth as the semiconductor body 2.

In greater detail, the buried regions 20 are separated from one another.In addition, in top plan view, the buried regions 20 may for examplehave an elongated shape (for instance, in a direction parallel to thefirst surface S_(a)), or else, once again by way of example, a shapechosen from between circular and polygonal. In general, in top planview, the buried regions 20 may thus define a strip-like or elsecell-like arrangement.

In even greater detail, the conductive layer 12 and the first epitaxiallayer 6 are such that a so-called Schottky contact is formed betweenthem. In the contact regions between the buried regions 20 and the firstepitaxial layer 6 there PN junctions are, instead, formed. In otherwords, the first epitaxial layer 6 and the buried regions 20 form,respectively, cathode regions and anode regions of bipolar diodes, whilethe first conductive layer 12 and the first epitaxial layer 6 form,respectively, an anode region and a cathode region of a Schottky diode.In use, the Schottky contact is activated for low biasing voltages ofthe diode 1, whereas the PN contacts are activated at higher voltages.Furthermore, since the buried regions 20 are separate from theconductive layer 12, the Schottky contact develops over a particularlyextensive area.

As illustrated in FIG. 2, further possible are embodiments in which thesemiconductor body 2 comprises a second epitaxial layer 24, arranged onthe first epitaxial layer 6.

For instance, the second epitaxial layer 24 may have approximately thesame doping level as the first epitaxial layer 6. In the case where,instead, the first and second epitaxial layers 6, 24 have differentdoping levels, two degrees of freedom are available for optimizing, inthe design stage, the compromise between forward-biasing voltage dropand reverse-biasing leakage current. Furthermore, the second epitaxiallayer 24 forms the aforementioned first surface S_(a) and a thirdsurface S_(c). In particular, the second epitaxial layer 24 overlies, indirect contact, the buried regions 20, which extend into the firstepitaxial layer 6 starting from the third surface S_(c). In practice,the third surface S_(c) delimits the first epitaxial layer 6 at the top.

FIG. 3 shows an additional embodiment, in which the diode 1 furthercomprises a plurality of regions 28, referred to in what follows as theconnection regions 28.

In detail, the connection regions 28 are of a conductive material, suchas for example nickel silicide, titanium silicide, molybdenum silicide,or a silicide of a transition metal. Furthermore, each connection region28 extends into the first epitaxial layer 6 starting from the firstsurface S_(a) until it contacts a corresponding buried region 20. Inaddition, as illustrated precisely in FIG. 3, each connection region 28may extend at least partially into the corresponding buried region 20,and in particular into a top portion of this buried region 20.

In greater detail, the connection regions 28 and the buried regions 20are such that the area of contact between each connection region 28 andthe corresponding buried region 20 forms a corresponding ohmic contact.In this way, the diode 1 is characterized by a particular strength inforward biasing.

As illustrated in FIG. 4, further possible are embodiments whereby thediode 1 includes both the connection regions 28 and the second epitaxiallayer 24. In this case, the connection regions 28 extend through thesecond epitaxial layer 24. In particular, without this implying any lossof generality, each top region 28 comprises a top portion, which extendsfrom the first surface S_(a) through the second epitaxial layer 24, anda bottom portion, which extends into the corresponding buried region 20.

The diode 1 illustrated in FIG. 1 can be obtained, for example, as shownin FIGS. 5A-5E and described in detail hereinafter.

Initially, as illustrated FIG. 5A, the semiconductor body 2, formed bythe substrate 4 and by the first epitaxial layer 6, is provided.

Next, as shown in FIG. 5B, formed on the first surface S_(a) is a hardmask 32, which defines a plurality of windows 34. In addition, using thewindows 34, an implantation of a P type is carried out, representedschematically by arrows 36, for example of aluminum ions. Theimplantation is carried out at high energy (for example, higher than 200keV) and with a dose ranging for example between 1·10¹³ and 2·10¹⁵atoms/cm². Further, this implantation leads to formation of regions 40of a buried type, i.e., far from the first surface S_(a). These regions40, referred to in what follows as the preliminary regions 40, are toform the buried regions 20.

Next, as shown in FIG. 5C, the hard mask 32 is removed. Furthermore, athermal process of activation of the implanted ions is carried out. Thisprocess is carried out at a temperature higher than 1500° C. Followingupon the thermal-activation process, each preliminary region 40 forms acorresponding buried region 20.

Next, as shown in FIG. 5D, deposited on the first surface S_(a) is amasking layer 42, made, for example, of TEOS oxide. Further, the bottomcontact layer 10 is formed underneath the second surface S_(b) and indirect contact therewith, in a per se known manner and consequently notillustrated. For instance, a bottom layer of metal material (forexample, nickel) is formed underneath the second surface S_(b) and indirect contact therewith, and then a thermal process is carried out,which causes a silicidation of the aforementioned bottom layer of metalmaterial, with consequent formation of the bottom contact layer 10.

Next, as shown in FIG. 5E, the masking layer 42 is removed. In addition,the conductive layer 12 is formed on the first surface S_(a), forexample by sputtering or evaporation.

Finally, in a way not illustrated, the top metallization 14 is formed onthe conductive layer 12, for example by sputtering or evaporation.

As regards the embodiment illustrated in FIG. 2, it can be obtained, forexample, in the way described hereinafter.

After the operations illustrated in FIG. 5A have been carried out, thehard mask 32 is formed, and an implantation of a P type, representedschematically by arrows 46, is carried out, as shown in FIG. 6A. Theimplantation is carried out at low energy (for instance, less than 200keV), for example with aluminum ions, and with a dose ranging, forexample, between 1·10¹³ and 2·10¹⁵ atoms/cm². Furthermore, thisimplantation leads to formation of the preliminary regions 40, whichgive out onto the top surface of the first epitaxial layer 6.

Next, as shown in FIG. 6B, the hard mask 32 is removed, and then aprocess of epitaxial growth is carried out for forming the secondepitaxial layer 24.

Next, as shown in FIG. 6C, a thermal process of activation of theimplanted ions is carried out. This process is carried out at atemperature higher than 1500° C. Following upon the thermal-activationprocess, each preliminary region 40 forms a corresponding buried region20.

Next, as shown in FIG. 6D, on the second epitaxial layer 24, and thus incontact with the first surface S_(a) of the semiconductor body 2, themasking layer 42 is deposited. Furthermore, the bottom contact layer 10is formed underneath the second surface S_(b) and in direct contacttherewith, for example in the way described previously.

Next, as shown in FIG. 6E, the masking layer 42 is removed. Furthermore,the conductive layer 12 is formed on the second epitaxial layer 24, forexample by sputtering or evaporation. Next, as described previously, thetop metallization 14 is formed.

As regards the embodiment shown in FIG. 3, it can be obtained in the waydescribed hereinafter.

Initially the operations illustrated in FIGS. 5A-5D are carried out.

Next, as shown in FIG. 7A, a plurality of trenches 50 is formed, forexample by a dry anisotropic etch. In particular, the trenches 50 areformed by a digging operation that entails selective removal of portionsof the masking layer 42 and of the first epitaxial layer 6.

In greater detail, each trench 50 extends from the top surface(designated by S_(d)) of the masking layer 42 and traverses, beyond themasking layer 42, a corresponding portion of the first epitaxial layer6, arranged between the masking layer 42 and a corresponding buriedregion 20, until it extends in part into said corresponding buriedregion 20. In particular, a bottom portion of each trench 50 extendsthrough a top portion of the corresponding buried region 20.Consequently the bottom of each trench 50 extends into the correspondingburied region 20.

Next, as shown in FIG. 7B, formed on the masking layer 42 and within thetrenches 50 is a layer 54 of metal material (for example, nickel,titanium or molybdenum), referred to in what follows as the fillinglayer 54. The filling layer 54 is obtained, for example, by sputteringor evaporation and fills the trenches 50 completely.

Next, as shown in FIG. 7C, a thermal process is carried out at atemperature comprised between 600° C. and 1100° C., and for a durationcomprised between 10 and 300 minutes, during which the portions of thefilling layer 54 arranged in contact with the semiconductor materialundergo a silicidation process, at the end of which they formcorresponding connection regions 28. The portion of filling layer 54that does not undergo any reaction is instead designated by 55 in FIG.7C.

As shown in FIG. 7D, the non-reacted portion 55 of the filling layer 54is then removed.

Next, as shown in FIG. 7E, the masking layer 42 is removed; further, theconductive layer 12 is formed on the first epitaxial layer 6. Next, in away not illustrated, the top metallization 14 is formed.

As regards the embodiment illustrated in FIG. 4, it can be obtained, forexample, in the way described hereinafter.

Initially, the operations illustrated in FIGS. 6A-6D are carried out.

Next, as shown in FIG. 8A, the trenches 50 are formed, for example onceagain by a dry anisotropic etch. In particular, the trenches 50 areformed by a digging operation that involves selective removal ofportions of the masking layer 42 and of the second epitaxial layer 24.

In greater detail, each trench 50 extends from the top surface(designated by S_(d)) of the masking layer 42 and traverses, beyond themasking layer 42, a corresponding portion of the second epitaxial layer24, arranged between the masking layer 42 and a corresponding buriedregion 20, until it extends in part into said corresponding buriedregion 20. In particular, a bottom portion of each trench 50 extendsthrough a top portion of the corresponding buried region 20.Consequently the bottom of each trench 50 extends into the correspondingburied region 20.

Next, as shown in FIG. 8B, the filling layer 54 is formed on the maskinglayer 42 and within the trenches 50.

Next, as shown in FIG. 8C, a thermal process is carried out at atemperature comprised between 600° C. and 1100° C., and for a durationcomprised between 10 and 300 minutes, during which the portions of thefilling layer 54 arranged in contact with the semiconductor materialundergo a silicidation process, at the end of which they formcorresponding connection regions 28. The portion of filling layer 54that does not undergo any reaction is instead designated by 55 in FIG.8C.

As shown in FIG. 8D, the non-reacted portion 55 of the filling layer 54is subsequently removed.

Next, as shown in FIG. 8E, the masking layer 42 is removed. Further, theconductive layer 12 is formed on the second epitaxial layer 24. Next, ina way not illustrated, the top metallization 14 is formed.

The switching device described presents numerous advantages. Inparticular, it can be shown that the present switching device presentssubstantially the same electrical field as a so-called JBS trench diode;i.e., it has a value of electrical field lower than the one set up inplanar JBS structures, but has a wider Schottky-contact area and thushas a wider useful area for passage of current. Furthermore, the presentswitching device is characterized by lower leakage currents, as well asby a forward-biasing voltage drop lower than what occurs, for example,in planar JBS structures.

Finally, it is clear that modifications and variations can be made tothe device and to the manufacturing method described and illustratedherein, without thereby departing from the scope of the presentdisclosure.

For instance, the types of doping may be reversed with respect to whathas been described herein.

As regards the manufacturing process, the order of the steps may bedifferent from what has been described herein. In addition, themanufacturing process may include further steps other than the onesdescribed. For instance, the manufacturing process may include, in a perse known manner, a so-called step of definition of the active area,which envisages formation of field-oxide regions (not illustrated)delimiting the area in which the JBS diode is to be obtained.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

The invention claimed is:
 1. A switching device comprising: a body ofsemiconductor material, which has a first conductivity type and a frontsurface; a contact layer of a first conductive material, which extendsin contact with the front surface; and a plurality of buried regions,which have a second conductivity type, are arranged within thesemiconductor body, and are spaced apart from the front surface, thecontact layer, and each other by semiconductor material portions of thebody.
 2. The switching device according to claim 1, further comprising aplurality of connection regions of a second conductive material, whichis different from the first conductive material, each connection regionextending in the semiconductor body from the front surface, andcontacting a corresponding buried region of the buried regions.
 3. Theswitching device according to claim 2, wherein each connection regionextends at least in part into the corresponding buried region.
 4. Theswitching device according to claim 2, wherein the semiconductor bodyand the contact layer form a Schottky contact; and wherein eachconnection region forms an ohmic contact with the corresponding buriedregion.
 5. The switching device according to claim 2, wherein saidsecond conductive material is a silicide of a transition metal.
 6. Theswitching device according to claim 1, wherein said first conductivematerial is a transition metal.
 7. The switching device according toclaim 1, wherein the body is of silicon carbide.
 8. The switching deviceaccording to claim 1, wherein the first conductivity type is an N typeand the second conductivity type is a P type.
 9. The switching deviceaccording to claim 1, further comprising a top layer of conductivematerial, which overlies, in direct contact, the contact layer and has agreater thickness than the contact layer.
 10. The switching deviceaccording to claim 1, which forms a Junction-barrier Schottky diode. 11.A process for manufacturing a switching device, comprising: forming,within a body of semiconductor material having a first conductivity typeand a front surface, a plurality of buried regions of a second type ofconductivity, said buried regions being spaced apart from the frontsurface and each other by semiconductor material portions of the body;and forming, in contact with the front surface, a contact layer of afirst conductive material.
 12. The process according to claim 11,further comprising forming a plurality of connection regions of a secondconductive material, which is different from the first conductivematerial, each connection region extending in the semiconductor bodyfrom the front surface and contacting a corresponding buried region ofthe plurality of buried regions.
 13. The process according to claim 12,wherein said forming the plurality of connection regions includesextending each connection region at least in part within thecorresponding buried region.
 14. The process according to claim 11,wherein said forming the plurality of buried regions comprises: carryingout an ion implantation that forms in the semiconductor body a pluralityof preliminary regions of a buried type; and carrying out a thermalprocess of activation of the ions of the preliminary regions.
 15. Theprocess according to claim 14, further comprising: forming a pluralityof connection regions of a second conductive material, which isdifferent from the first conductive material, each connection regionextending in the semiconductor body from the front surface andcontacting a corresponding buried region of the plurality of buriedregions; forming a masking layer of dielectric material on top of saidfront surface; forming a plurality of trenches so that each trenchtraverses the masking layer, a portion of the semiconductor body, and atleast part of a corresponding buried region of the plurality of buriedregions; filling the trenches with filling regions of a third conductivematerial; getting portions of the filling regions to react thermallywith the semiconductor material of the semiconductor body and form asilicide; and removing portions of the filling regions that have notreacted thermally with the semiconductor material of the semiconductorbody.
 16. The process according to claim 11, wherein said forming theplurality of buried regions comprises: carrying out an ion implantationthat forms in the semiconductor body a plurality of preliminary regionshaving a temporary surface that temporarily delimits said semiconductorbody; and growing an epitaxial layer on said temporary surface; saidprocess further comprising carrying out a thermal process of activationof the ions of the preliminary regions.
 17. The process according toclaim 16, further comprising: forming a masking layer of dielectricmaterial on top of said front surface; forming a plurality of trenchesso that each trench traverses the masking layer, a portion of thesemiconductor body, and at least part of a corresponding buried regionof the plurality of buried regions; filling the trenches with fillingregions of a third conductive material; getting portions of the fillingregions to react thermally with the semiconductor material of thesemiconductor body and form a silicide; and removing portions of thefilling regions that have not reacted thermally with the semiconductormaterial of the semiconductor body.
 18. A Junction-barrier Schottkydiode comprising: a body of semiconductor material, which has a firstconductivity type and a front surface; a contact layer of a firstconductive material, which extends in contact with the front surface; aburied region, of a second conductivity type, arranged within thesemiconductor body, and spaced apart from the front surface and thecontact layer by a portion of the body; and a connection region of asecond conductive material, which is different from the first conductivematerial, the connection region extending in the semiconductor body fromthe front surface and contacting the buried region.
 19. TheJunction-barrier Schottky diode according to claim 18, wherein thesemiconductor body and the contact layer form a Schottky contact; andwherein each connection region forms an ohmic contact with thecorresponding buried region.
 20. The Junction-barrier Schottky diodeaccording to claim 18, further comprising a top layer of conductivematerial, which overlies, in direct contact, the contact layer and has agreater thickness than the contact layer.